diff --git a/python/cudf_polars/cudf_polars/dsl/expressions/rolling.py b/python/cudf_polars/cudf_polars/dsl/expressions/rolling.py
index e8cbf0c3f25..df039d89283 100644
--- a/python/cudf_polars/cudf_polars/dsl/expressions/rolling.py
+++ b/python/cudf_polars/cudf_polars/dsl/expressions/rolling.py
@@ -48,6 +48,11 @@ class FillNullWithStrategyOp(UnaryOp):
     policy: plc.replace.ReplacePolicy = plc.replace.ReplacePolicy.PRECEDING
 
 
+@dataclass(frozen=True)
+class CumSumOp(UnaryOp):
+    pass
+
+
 def to_request(
     value: expr.Expr, orderby: Column, df: DataFrame
 ) -> plc.rolling.RollingRequest:
@@ -241,7 +246,8 @@ def __init__(
                 isinstance(named_expr.value, (expr.Len, expr.Agg))
                 or (
                     isinstance(named_expr.value, expr.UnaryFunction)
-                    and named_expr.value.name in {"rank", "fill_null_with_strategy"}
+                    and named_expr.value.name
+                    in {"rank", "fill_null_with_strategy", "cum_sum"}
                 )
             )
         ]
@@ -265,7 +271,7 @@ def __init__(
             if isinstance(v, expr.Agg)
             or (
                 isinstance(v, expr.UnaryFunction)
-                and v.name in {"rank", "fill_null_with_strategy"}
+                and v.name in {"rank", "fill_null_with_strategy", "cum_sum"}
             )
         ]
         self.by_count = len(by_expr)
@@ -393,6 +399,41 @@ def _(
         dtypes = [ne.value.dtype for ne in named_exprs]
         return names, dtypes, tables
 
+    @_apply_unary_op.register
+    def _(
+        self,
+        op: CumSumOp,
+        df: DataFrame,
+        _: plc.groupby.GroupBy,
+    ) -> tuple[list[str], list[DataType], list[plc.Table]]:
+        cum_named = op.named_exprs
+        order_index = op.order_index
+
+        requests: list[plc.groupby.GroupByRequest] = []
+        out_names: list[str] = []
+        out_dtypes: list[DataType] = []
+
+        val_cols = self._gather_columns(
+            [
+                ne.value.children[0].evaluate(df, context=ExecutionContext.FRAME).obj
+                for ne in cum_named
+            ],
+            order_index,
+            cudf_polars_column=False,
+        )
+        agg = plc.aggregation.sum()
+
+        for ne, val_col in zip(cum_named, val_cols, strict=True):
+            requests.append(plc.groupby.GroupByRequest(val_col, [agg]))
+            out_names.append(ne.name)
+            out_dtypes.append(ne.value.dtype)
+
+        lg = op.local_grouper
+        assert isinstance(lg, plc.groupby.GroupBy)
+        _, tables = lg.scan(requests)
+
+        return out_names, out_dtypes, tables
+
     def _reorder_to_input(
         self,
         row_id: plc.Column,
@@ -444,6 +485,7 @@ def _split_named_expr(
         unary_window_ops: dict[str, list[expr.NamedExpr]] = {
             "rank": [],
             "fill_null_with_strategy": [],
+            "cum_sum": [],
         }
 
         for ne in self.named_aggs:
@@ -733,6 +775,40 @@ def do_evaluate(  # noqa: D102
                     )
                 )
 
+        if cum_named := unary_window_ops["cum_sum"]:
+            order_index = self._build_window_order_index(
+                by_cols,
+                row_id=row_id,
+                order_by_col=order_by_col if self._order_by_expr is not None else None,
+                ob_desc=self.options[2] if self._order_by_expr is not None else False,
+                ob_nulls_last=self.options[3]
+                if self._order_by_expr is not None
+                else False,
+            )
+            by_cols_for_scan = self._gather_columns(by_cols, order_index)
+            local = self._sorted_grouper(by_cols_for_scan)
+            names, dtypes, tables = self._apply_unary_op(
+                CumSumOp(
+                    named_exprs=cum_named,
+                    order_index=order_index,
+                    by_cols_for_scan=by_cols_for_scan,
+                    local_grouper=local,
+                ),
+                df,
+                grouper,
+            )
+            broadcasted_cols.extend(
+                self._reorder_to_input(
+                    row_id,
+                    by_cols,
+                    df.num_rows,
+                    tables,
+                    names,
+                    dtypes,
+                    order_index=order_index,
+                )
+            )
+
         # Create a temporary DataFrame with the broadcasted columns named by their
         # placeholder names from agg decomposition, then evaluate the post-expression.
         df = DataFrame(broadcasted_cols)
diff --git a/python/cudf_polars/cudf_polars/dsl/utils/aggregations.py b/python/cudf_polars/cudf_polars/dsl/utils/aggregations.py
index 6e1eefb298f..5699dc639bd 100644
--- a/python/cudf_polars/cudf_polars/dsl/utils/aggregations.py
+++ b/python/cudf_polars/cudf_polars/dsl/utils/aggregations.py
@@ -92,6 +92,7 @@ def decompose_single_agg(
     if isinstance(agg, expr.UnaryFunction) and agg.name in {
         "rank",
         "fill_null_with_strategy",
+        "cum_sum",
     }:
         if context != ExecutionContext.WINDOW:
             raise NotImplementedError(
diff --git a/python/cudf_polars/tests/expressions/test_rolling.py b/python/cudf_polars/tests/expressions/test_rolling.py
index 6abe7af1a14..dd01511c85e 100644
--- a/python/cudf_polars/tests/expressions/test_rolling.py
+++ b/python/cudf_polars/tests/expressions/test_rolling.py
@@ -398,3 +398,29 @@ def test_fill_over(
 def test_fill_null_with_mean_over_unsupported(df: pl.LazyFrame) -> None:
     q = df.select(pl.col("x").fill_null(strategy="mean").over("g"))
     assert_ir_translation_raises(q, NotImplementedError)
+
+
+@pytest.mark.parametrize(
+    "expr,group_key",
+    [
+        (pl.col("x"), "g"),
+        (pl.when((pl.col("x") % 4) == 1).then(None).otherwise(pl.col("x")), "g"),
+        (pl.col("x"), "g_null"),
+    ],
+)
+@pytest.mark.parametrize(
+    "order_by",
+    [
+        None,
+        ["g2", pl.col("x2") * 2],
+    ],
+)
+def test_cum_sum_over(
+    df: pl.LazyFrame,
+    *,
+    expr: pl.Expr,
+    group_key: str,
+    order_by: None | list[str | pl.Expr],
+) -> None:
+    q = df.select(expr.cum_sum().over(group_key, order_by=order_by))
+    assert_gpu_result_equal(q)